Engine exhaust system including internal expansion chambers around the exhaust branch pipes

ABSTRACT

A snowmobile having a lightweight fabricated frame assembly. The frame assembly includes a number of reinforcing elements, some of which comprise heat exchangers for the liquid cooling system of the engine, so as to provide high rigidity and light weight. The engine of the snowmobile is provided with an exhaust system that permits a relatively short effective length, but minimizes pulse-back effects by forming internal expansion chambers around the exhaust branch pipes. The drive for the snowmobile includes a reversing transmission and a brake assembly associated therewith in addition to a main brake assembly for the snowmobile. Furthermore, there is provided a parking brake assembly that is easy to operate and which will provide the operator with a visual and physical warning of its condition so that the operator will not attempt to operate the snowmobile with the parking brake engaged.

BACKGROUND OF THE INVENTION

This invention relates to an internal combustion engine exhaust systemand more particularly to an improved high performance exhaust system foran engine.

With internal combustion engines, the exhaust system is quiteinfluential in affecting the performance of the engine. By appropriatelysizing the length and cross-sectional flow area of the exhaust system,the engine power curve can be significantly altered. With multi-cylinderengines, the utilization of a common exhaust pipe for several cylinderscan give rise to effects on the exhaust pulses from one cylinder toanother. This is particularly true but not limited to two-cycle internalcombustion engines because of there more frequent firing.

The exhaust pulses that exit the tailpipe end of the exhaust pipe arereflected back through the exhaust system to the exhaust ports of theengine. Thus, if the exhaust system is not properly designed, adverseperformance can result. This problem is particularly relevant to certaintypes of vehicles which are compact in nature and which dictate fromtheir configuration the configuration and length of the exhaust system.

In order to provide high performance, the exhaust system should berelatively short. However the use of short exhaust systems particularlywhen one exhaust pipe serves several cylinders, can result in sacrificesin power and performance at other engine speeds and loads. For example,a high speed exhaust system for a three-cylinder engine and particularlya two-cycle type engine can cause exhaust pulses that are transmittedback to other cylinders than that that is in the exhaust cycle. This cancause adverse effects on the scavenging. This is particularly a problemwith two-cycle engines because of the relatively high degree of overlapbetween the scavenging port and the exhaust port, and the fact thatthese engines fire every crankshaft revolution. The same circumstances,however, also can apply to four-cycle engines.

It is, therefore, a principal object of this invention to provide animproved exhaust system for a multi-cylinder engine.

It is a further object of this invention to provide an improved exhaustsystem for a multi-cylinder engine that will improve the performancethroughout the entire engine speed and load ranges.

It is a still further object of this invention to provide an improvedand compact exhaust system for a multi-cylinder engine whereinpulse-back effects from one cylinder will not adversely affect othercylinders.

SUMMARY OF THE INVENTION

This invention is adapted to be embodied in an exhaust system for aninternal combustion engine having at least two combustion chambersformed with exhaust ports through which the exhaust gasses aredischarged from the combustion chambers to the atmosphere. The exhaustsystem comprises a pair of spaced apart tubular branch pipes that extendfrom the exhaust ports to a common atmospheric discharge end. Means areaffixed to the exhaust branch pipes and defined therewith a closedexpansion chamber in direct surface contact with portions of each of theexhaust branch pipes. A passage extends through at least one of theportions of one of the exhaust branch pipes for providing communicationof the exhaust gasses from the served combustion chamber into theexpansion chamber for minimizing pulse-back effects on anothercombustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a snowmobile constructed inaccordance with the invention.

FIG. 2 is an enlarged top plan view showing the forward portion of thesnowmobile.

FIG. 3 is a side elevational view of the frame assembly for thesnowmobile on a slightly larger scale than that of FIG. 1.

FIG. 4 is a top plan view, in part similar to FIG. 2, but with thesnowmobile body broken away and portions of the frame shown in sectionso as to illustrate a portion of the steering mechanism for the frontskis.

FIG. 5 is an enlarged side elevational view looking in the samedirection of FIG. 3, but showing only the front portion of the frame.

FIG. 6 is a top plan view of the front frame portion on the same scaleas FIG. 5.

FIG. 7 is a side elevational view of the same components shown in FIG.4, but on a slightly smaller scale, to illustrate the steering mechanismfor the front skis.

FIG. 8 is an enlarged rear elevational view of the front bulkheadcasting for the front frame sub assembly.

FIG. 9 is a side elevational view of the front frame bulkhead casting.

FIG. 10 is a top plan view of the exhaust manifold and exhaust systemfor the engine with only the exhaust system mounting components of thesnowmobile frame illustrated.

FIG. 11 is a cross-sectional view of the exhaust manifold taken alongthe line 11--11 of FIG. 12.

FIG. 12 is a cross-sectional view of the exhaust manifold takengenerally along the line 12--12 of FIG. 11.

FIG. 13 is a graphical view showing the power curve of an engineconstructed in accordance with this embodiment of the invention in solidlines and in accordance with the prior art in broken lines.

FIG. 14 is a view with portions broken away showing the reversingtransmission and control mechanisms therefor.

FIG. 15 is an enlarged cross-sectional view through the reversingtransmission mechanism taken along the line 15--15 of FIG. 14.

FIG. 16 is a view looking in the direction opposite that of FIG. 14, butshowing only the reversing transmission control actuating lever system.

FIG. 17 is a side elevational view showing the brake mechanismassociated with the transmission control and also with the parkingbrake.

FIG. 18 is a partial cross-sectional view showing the shifting fork ofthe transmission mechanism and the actuating portion therefor.

FIG. 19 is a view looking in the same direction as FIG. 14, but showingthe association of the transmission to the battery for the vehicle andillustrating the battery mounting arrangement.

FIG. 20 is a top plan view showing the handlebar and the controlsassociated therewith.

FIG. 21 is a top plan view showing the parking rake control and itsassociation with the parking brake which has been shown out of positionto illustrate it in this figure.

FIG. 22 is a side elevational view showing the engine and its coolingsystem with other portions of the snowmobile removed or shown inphantom.

FIG. 23 is a top plan view of the same assemblage shown in FIG. 21.

FIG. 24 is a cross-sectional view taken through and showing the frontradiator on a larger scale and taken along the line 23--23 of both FIGS.22 and 24.

FIG. 25 is an enlarged top plan view of the front radiator.

FIG. 26 is a cross-sectional view taken along the line 25--25 of FIG. 22and shows one of the side radiators.

FIG. 27 is a top plan view, in part similar to FIG. 23 and shows anotherembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION TheOverall Snowmobile

Referring now in detail initially to FIGS. 1 and 2, a snowmobileconstructed in accordance with an embodiment of the invention isidentified generally by the reference numeral 31. Although the inventionrelates to an exhaust system for an engine, a snowmobile is depicted asa typical environment in which the invention may be practiced. This is atypical environment because the invention has particular utility withexhaust systems that are designed to improve the performance throughoutthe entire engine speed and load ranges and be capable of fitting in acompact area. Furthermore, the invention has particular utility, but isnot limited to, utilization with two-cycle engines of multiple-cylinderconfigurations. Such engines are frequently employed as the prime moversin snowmobiles. It will be apparent to those skilled in the art,however, how the invention can be utilized with a wide variety of typesof engines and applications for such engines.

The snowmobile 31 is comprised of a frame assembly, indicated generallyby the reference numeral 32. This frame assembly will be described laterin more detail by particular reference to FIGS. 3-9. However, the frameassembly 32 is comprised primarily of a fabricated rear framesubassembly 33, and a fabricated front frame subassembly, indicatedgenerally by the reference numeral 34. The front frame assembly 34includes a cast bulkhead 35 having a construction which will also bedescribed later by reference to FIG. 3-9 and specifically to FIGS. 8 and9.

A body cowling 36 is mounted over the forward portion of the front frameassembly 34 and cooperates with the rear frame assembly 33 so as to forma rider's area to the rear of a controlling handlebar assembly 37.Directly to the rear of the handlebar assembly 37 there is positioned afuel tank 38 and a rider's seat 39 is disposed to the rear of this fueltank. The area to the rear of the rider's seat 39 is partially elevatedat 41 so as to form a rear storage compartment 42 that is accessiblethrough an openable closure 43.

The body assembly 36 further includes a cowling or windshield 44disposed to the front of the handlebar assembly 37 in protectingrelationship to the rider or riders. A front bumper 45 is affixed to thefront of the body 36 via the frame assembly 32 in a manner which willalso be described.

A pair of front skis 46 are supported for suspension and dirigiblemovement at the front of the snowmobile 31 and specifically by the frontframe assembly 34 in a manner which also will be described in moredetail later. Basically, these skis 46 are supported within tubularmembers 47 by a suspension system which will be described and whichtubular members 47 permit steering movement of the skis 46 under controlof the handlebar assembly 37. As noted, this construction will bedescribed in more detail later.

A drive belt 48 is supported at the rear of the snowmobile 31 beneaththe seat 39 and specifically beneath an inverted U-shape portion of therear frame subassembly 33. This drive belt assembly 48 is supported atits lower surface by a pair of guide rails 49 that carry a plurality ofidler rollers 51 and a main rear idler roller 52.

An idler roller 53 is mounted by the frame assembly and also cooperatesto form the path for the drive belt 48. The drive belt 48 is supportedfrom the frame assembly 32 by a suitable suspension system which may beof any known type. This includes a front linkage 54 and associatedcushioning unit (not shown). A rear linkage assembly 55 and additionalcushion unit which is also not shown supports the rear end of the guiderails 49 and the drive belt 48.

Since the invention does not deal with the details of the suspensionsystem for either the front skis 46 or the drive belt 48, where anydetails of these suspensions systems are not illustrated or described,any conventional structure may be utilized as should be readily apparentto those skilled in the art.

An internal combustion engine, indicated generally by the referencenumber 56 is mounted in the frame assembly 32 and specifically the frontframe subassembly 34 in a manner which will be described. The engine 56is of the three-cylinder in-line type and operates on a two-strokecrankcase compression principle. As has been previously noted, theinvention has particular utility in conjunction with such engines, butis not limited to this specific type of engine.

The engine 56 drives the drive belt 48 in a manner which will bedescribed. However, this drive assembly includes a continuously variablebelt-type transmission, indicated generally by the reference numeral 57which is placed on the left-hand side of the snowmobile 31 as viewed bya seated operator. This drive belt assembly 57 drives an intermediateshaft 58 which is journaled in the frame assembly 32 in a manner whichalso will be described. A final drive 59 that includes a reversingtransmission drives the drive belt 48 through a drive belt shaft, to bedescribed later along with the reversing transmission by particularreference to FIGS. 14-16.

The Frame Assembly

The construction of the frame assembly 32 will now be described byparticular reference to FIGS. 3-9. As has been previously noted, theframe assembly 32 is made up of two primary frame subassembliescomprised of a rear frame subassembly 33 and a front frame subassembly34.

The rear frame subassembly 33 is comprised of a pressing or stampingmade from a sheet metal, such as steel, aluminum or aluminum alloy. Ithas a generally inverted U-shaped configuration that consists of ahorizontally extending planar upper portion 61 upon which the seat 39 issupported. The upper portion has a pair of depending side portions 62,each of which has an outwardly extending portion 63 and 64 that formfoot areas for accommodating the feet of a seated rider. These footareas 63 and 64 are generally coextensive in length with the seat 39 andextend rearwardly beyond the rear portion of the seat 39 to lie alongthe outer sides of the storage compartment 42.

At its forward end, the rear frame subassembly 33 is formed with anoutstanding flange portion 65 which is adapted to nest around theforward frame subassembly 34 and be affixed to it as by welding,riveting, or a combination thereof. By so forming the rear framesubassembly 33, the connection can be very robust and encompass a largesurface area. In addition, since there are no bent flanges, the strengthof the assembly is increased.

The front frame subassembly 34 is also constructed primarily from asheet metal pressing, indicated generally by the reference numeral 66and which has a U-shaped configuration that is comprised of a lower wallpart 67 extends generally horizontally, although it has a pair ofangularly related portions. A pair of upstanding side walls 68 and 69are formed integrally with this lower wall 67.

The side wall 68 is formed with a generally oval-shaped opening 71 whichaligned with the engine 56 and which facilitates passage therethrough aportion of the engine and drive train, as will become apparent.

The opposing side wall 69 is formed with a slotted opening 72 whichextends through its upper peripheral edge and which is sized so as tofacilitate insertion of the engine between the side members 68 and 69.The upper end of the opening 72 is closed by a closure plate 73 once theengine is in place. This closure plate 73 is detachably connected to theside wall 69, as by threaded fasteners or the like. By closing theopening 72, the side wall 69 is reinforced and, yet, the engine 56 canbe easily installed and removed.

An intermediate reinforcing member, indicated generally by the referencenumeral 74, extends between the side walls 68 and 69, and has flangeportions that are affixed thereto as by welding, riveting, or acombination thereof. This reinforcing member 74 is comprised of a rear,horizontally extending portion 75 which is generally aligned with therear frame upper wall portion 61. The portion 75 terminates at itsforward end in a downwardly sloped part 76 which forms a cradle intowhich the engine 56 can be positioned and upon which it can besupported.

A sub frame cross-member 77 extends forwardly of the wall portion 76 andis connected thereto, as well as to the side walls 68 and 69, and thelower wall 67, as by welding, riveting, or the like.

Openings 78 are formed in the forward portions of the side walls 68 and69, so as to provide lightening of the assembly and access to variouscomponents, as will be noted. This is done without reducing the overallstrength of the assembly.

As has been previously noted, the forward end of the front framesubassembly 34 is closed by a bulkhead, indicated generally by thereference numeral 35. This bulkhead 35 has a configuration as best seenin FIGS. 8 and 9.

The bulkhead 35 is formed primarily as a casting from aluminum oraluminum alloy. The upper surface of the bulkhead 35 is provided with anarcuate recess 79 that is defined by a U-shaped wall portion thereof.The U shaped recess is provided to clear the exhaust system, as will bedescribed later.

This U-shaped wall portion 79 is bounded by a pair of outer sidewalls 81and 82, along which the forward sides of the side walls 68 and 69 nest.These forward portions of the side walls 68 and 69 can thus be affixeddirectly to the bulkhead 35 by riveting, welding, or a combinationthereof. Alternatively, threaded fasteners may be utilized. In any ofthe these events, the structure provides a very good reinforcing and,again, because the joint is formed by planar surfaces, there will be nolocal stress risers that could weaken the overall construction.

These bulkhead sidewalls 81 and 82 have curved surfaces and the rearface of the bulkhead 35 is provided with a plurality of reinforcing ribs83 that run diagonally thereacross so as to add to the rigidity of thestructure without significantly increasing its weight. Also, since theribs 83 extend on the rear side of the bulkhead 35, they will not besubject to the accumulation of foreign material as if they had facedforwardly. The ribs 83 are formed on the protected side of the bulkhead35 and, thus, will be able to kept fairly clean by their own internalconstruction. That is, the ribs 83, by being formed on the rear side ofthe bulkhead, will not be subject to collection of debris as if theywere forwardly facing.

The sides of the bulkhead 35 are provided with a pair of embossments 84that are adapted to receive threaded fasteners for affixing a bumper subframe 85 thereto. This bumper sub frame 85 appears in most detail inFIGS. 2 and 3, and permits the front bumper 45 to be attached directlythereto around an intervening lower portion 86 of the body 36. Hence,the bumper 45 will be very robust and any forces exerted thereto will betransmitted directly to the frame assembly 32.

The Front Suspension

The suspension arrangement for the front skis 46 and for the steeringthereof will now be described by primary reference to FIGS. 2, 4 and 7.The front skis 46, as has been noted, are supported for steeringmovement by means a pair of tubes 47. These tubes 47 are supported forsuspension travel by a pair of suspension arms 87 which have a jointedconnection 88 to the tubular member 49 at their outer ends. The innerends of these suspension arms 87 are pivotally supported on the bulkhead35 by a pivotal support structure which is not shown, but which isaffixed to the bulkhead 35 by means of bosses 89 (FIG. 8) formed in thebulkhead assembly 35 and reinforced by the ribs 83.

In addition to the suspension arms 87, trailing arms 91 are affixed attheir forward ends to the tubular members 47. The rear ends of thetrailing arms 91 are pivotally connected to the frame assembly andspecifically to the rear portion of the front frame subassembly 34 inknown manner.

The upper ends of the pins which support the skis 46 for steeringmovement in the tubular members 47 have connected to them steering arms92. These steering arms 92 are controlled by tie rods 93 that have himejoints 94 at their outer ends for pivotal connection to the steeringarms 92. The inner ends of the tie rods 93 are connected by further himejoints 95 to a bell crank assembly 96 of the steering mechanism,indicated generally by the reference numeral 97. The steering bell crank96 is pivotally supported on a pivot shaft 98 which is affixed to a boss99 (FIGS. 6 and 8) of the bulkhead 35 at the center of the recess 79.

A drag link 101 is connected by a hime joint 102 to the bell crank 96 atits forward end. The rear end of the drag link 101 is connected by afurther hime joint 103 to a pivot arm 104 that is affixed to the lowerend of a steering shaft 105. The lower end of the steering shaft 105 isjournaled in a bearing assembly 106 that is mounted to the frameassembly 34 in a manner which will be described later. The upper end ofthe steering shaft 105 is journaled by a further bearing 107 that ismounted on an extension 108 of the frame assembly. The handlebarassembly 37 is affixed to the upper end of the steering shaft 105 in aknown manner for steering of the front skis 46.

It should be noted that the frame member 74 and specifically itsinclined portion 76 is formed with an enlarged opening 109 through whichthe steering mechanism can be readily accessed.

The Engine and Exhaust System

A portion of the construction of the engine 56 will now be described byprimary reference to FIGS. 1 and 2. As has been noted, the engine 56 is,in the illustrated embodiment, of the three-cylinder in-line type andoperates on a two-stroke crankcase compression principle. The engine 56is also liquid cooled and is comprised of a cylinder block 111 whichextends generally transversely across the frame assembly 32 as has beenpreviously described. This engine cylinder block 111 is supported bysuitable engine mounts in the cradle formed by the front framesubassembly 34 and specifically by the frame members 74 and 77. Thecylinder block 111 is formed with three transversely aligned cylinderbores that are closed by a cylinder head assembly 112 that is affixed tothe upper end of the cylinder block 111 in any well-known manner.

An air box 113 is mounted in the rear portion of the front framesubassembly 34 and is covered by the body cover 36. This air box 113delivers air to charge formers 114 which may be comprised of carburetorsthat serve intake ports of the engine through short connectingmanifolds.

The charge thus formed is delivered to the crankcase chambers of theengine for compression and transfer through scavenge passages to thecombustion chambers of the engine. This charge is then fired by sparkplugs 115 that are mounted in the cylinder head assembly 112. The sparkplugs 115 are fired by a spark control box 116 that is convenientlymounted on the upper portion of the air box 113 so as to be cooled bythe air flow therethrough.

The charge which is exhausted from the combustion chambers throughexhaust ports formed in the front of the cylinder block 111 anddelivered to an exhaust system, indicated generally by the referencenumeral 117, which is constructed in accordance with a feature of theinvention.

The exhaust system 117 is shown in most detail in FIGS. 10-12 and willbe described by reference thereto. This exhaust system includes anexhaust manifold assembly, indicated generally by the reference numeral118 and which embodies an important feature of the invention. Thisexhaust manifold assembly 118 is comprised of a pair of end runners 119and 121 and a center runner 122. Each runner 119, 121, and 122 has thesame general construction. That is, the runners 119, 121, and 122 areformed by semi-cylindrical portions that have flanges that are weldedtogether to form the tubular shape thereof as best seen in FIG. 11.

The end runners 119 and 121 join with the center runner 122 close to thedischarge end thereof. A collector pipe 123 is joined at the dischargeend thereof. This collector pipe 123 has a flanged connection 124 to anexpansion chamber device 125. This joint 124 is held together by aspring-type clamp assembly 126.

The expansion chamber device 125 generally curves around an area toclear the steering shaft 115 and is supported on the upper side of thebulkhead 35 within the recess 79. The expansion chamber device 125, inturn, has its discharge end connected by a further spring-held flangeconnection 127 to a muffler 128. The muffler 128 is supported upon aboss portion 129 of the bulkhead 35 and held in place to the framesubassembly 34 by a spring clip 129. Exhaust gases are discharged to theatmosphere through a suitable exhaust pipe.

Obviously, the exhaust manifold 118 is quite compact and relativelyshort in length. This is done so as to provide a high degree of exhaustefficiency and good tuning for good high-speed performance. However,because of the fact that the end branch pipes 119 and 121 are connectedto the center branch pipe 122 in close proximity and in close proximityto the engine exhaust ports, the engine may experience pulse-backeffects that inhibit the charging efficiency and thus can provide dipsin the power curve as best seen in FIG. 13.

The dotted line view of this figure shows the problem of loss oflow-speed power with a conventional system having a construction as thusfar described. In accordance with the invention, this falloff in engineperformance at low speeds is avoided by providing a pair of expansionchambers, indicated by the reference numerals 131 and 132, best shown inFIGS. 11 and 12 and which have a restricted communication with thebranch pipes 119 and 122 and 122 and 121.

These expansion chambers 131 and 132 are easily formed by top plates 133and bottom plates 134 that are affixed, as by welding, to the upper andlower tubular portions of the pipe branches 119, 122, and 121. Thevolume of these expansion chambers 131 and 132 is adjusted by providinga vertically extending closure plate 135 across the rearward end of eachset of plates 133 and 134.

Each of the end branch pipes 119 and 121 is provided with a respectiverelatively small communication opening 136 and 137 that is disposed inthe angular area θ₁ formed by the juncture of the branch pipes 119 and122 and 122 and 121. In a like manner, the center branch pipe 122 isformed with a pair of openings 138 and 139 so that the openings 136,138, 137, and 139 are in this angular area θ₁. This angular area θ₁ isgreater than the angular area θ₂ formed where the branch pipes 119 and121 are joined by the plates 135 so as to afford free access to thisarea of the exhaust gases.

By providing these small expansion chambers 131 and 132 it is possibleto not only reduce the dip in the power curve that occurs a low speedbut also to improve the power throughout the entire engine speed rangeas shown by the solid line curve in FIG. 13.

Finally, the cylinder block side of the branch pipes 119, 121, and 122is reinforced by a flange plate 141 that is provided with apertures sothat it can be affixed to the cylinder block 111 of the engine 56.

The Transmission

The arrangement by which the engine 56 drives the drive belt 48 will nowbe described by reference to FIGS. 1, 2, 14 and 15. As has beenpreviously noted, this transmission includes the continuously variabletransmission (CVT) indicated generally by the reference numeral 57. Thiscontinuously variable transmission 57 includes a driving pulley 142which is driven from the engine crankshaft 143 via a centrifugal clutch.The variable driving pulley 142 drives a drive belt 143 which, in turn,drives a driven variable pulley 144.

The driven pulley 144 is associated with the intermediate shaft 58. Thisintermediate shaft 58 extends transversely across the width of thesnowmobile frame 32 and terminates at an end that is journaled in ananti-friction bearing 145. The bearing 145 is supported within atransmission casing, indicated generally by the reference numeral 146that comprises a first member 147 that is affixed to the frame side wall69. This defines a cavity into which the shaft 58 extends. This cavityis closed by a cover plate 148 that is affixed to the plate 147 with aseal 149 formed therebetween.

The end of the intermediate drive shaft 58 that extends into thetransmission case 146 has affixed to it a driving sprocket 151. Thedriving sprocket 151 has a key connection to the intermediate shaft 58and is held thereon by a retaining nut 152.

This sprocket 151 drives a drive chain 153 which, as best seen in FIG.14, is engaged with a reverse drive sprocket 154 and a forward drivesprocket 155. Both the reverse drive sprocket 154 and the forward drivesprocket 155 rotate in the same direction. However, they are coupled ina different manner to a belt driving shaft 156 which forms the outputshaft of the final drive reversing transmission, which transmission isindicated as noted generally by the reference numeral 59 and whichincludes a reversing drive, as will be described.

Lubricant may be received in the transmission case 146 for lubricatingthe driving components. The lubricant level may be checked by a dipstick 160.

The reverse drive sprocket 154 is journaled on a stub shaft 157 which ismounted in the casing between the members 147 and 148. The forwarddriving sprocket 155 is rotatably journaled by means of a needle-bearingassembly 158 on the belt driving shaft 156. In this regard, it should benoted that the belt driving shaft 156 extends through the forward end ofthe drive belt 48 and has one or more lugs on it which are engaged withcorresponding lugs of the drive belt 48 to drive it in a manner wellknown in this art.

The belt-driving shaft 156 is journaled at one end by a bearing 159 thatis fixed to the transmission case 147. The other end of the belt drivingshaft 156 may be journaled in a similar bearing carried at the oppositeside of the frame assembly 32 and which is not shown.

The reverse drive gear 154 is coupled for rotation with a furtherreverse drive gear 161, as by drive pins 162. The second reverse drivegear 161 is also journaled on the stub shaft 157 by needle bearings,indicated by the reference numeral 163.

A combined gear and dog clutch member, indicated generally by thereference numeral 164 is splined for rotation with the belt drive shaft156 outwardly of the forward driving sprocket 155. Actually, the reversegear and dog clutching member 164 is splined on a tubular stub shaft 165which in turn has a splined connection to the belt drive shaft 156inwardly of the needle bearing 158 and which is held axially in place bya retainer element 166 and nut 167.

In the forward drive mode, as shown in the solid line view of FIG. 15,dog clutching elements 168 are engaged with the forward drive sprocket155 so as to establish a driving relationship between this sprocket, thereverse drive gear and dog clutching member 164 and the belt drivingshaft.

When the reverse driving gear and dog clutching member 164 is shifted tothe right as shown in FIG. 15 to the phantom line position, the teeth169 of the gear 164 will be brought into meshing relationship with theteeth of the second reverse drive gear 161. At this time, the gear 164,stub shaft 165, and belt driving shaft 156 will be driven in an oppositedirection or the reverse drive direction. Hence, a very compact buthighly effective forward reverse transmission is provided by thismechanism.

The Transmission Control

The shifting mechanism by which this transmission reversal is obtainedwill now be described by primary reference to FIGS. 14-16, and 18. Theshifting of the transmission 59 between its forward drive mode and itsreverse drive mode is controlled by an operator control shift lever,indicated generally by the reference numeral 171 and which is mounted onthe right-hand side of the vehicle in close proximity to the handlebar37.

This shifting mechanism includes a shift rod 172 that carries a shiftknob 173 at one end thereof. The shift rod 172 is connected at itsopposite end, as by welding, to a shift bracket 174. The shift bracket174 is pivotally connected about a pivot pin 175 to a shift lever 176.The pivotal movement of the bracket 174 about the shift lever 176 isutilized to provide a release for a locking mechanism which locks theshift lever assembly 171 in either its forward and reverse drivepositions. In addition, this pivotal movement is also utilized toactuate a brake, as will be described, so as to retard the rotation ofthe intermediate shaft 58 so as to facilitate smooth shifting. Thisbraking action occurs temporarily during the movement between theforward drive position and the reverse drive position as shown in FIG.14 and which motion will be described in more detail later after thetotal mechanism is described.

The shift lever 176 is disconnectedly connected to a further bracketmechanism 177 which is pivotally supported on a mounting bracket 178 bymeans of a further pivot pin 179. The bracket 178 is convenientlyaffixed to an appropriate portion of the frame assembly. A tensionspring 181 is interlocked between the shift bracket 174 and the plate177 and normally biases the mechanism to the position wherein the shiftrod 172 is pivoted to an upward location as shown in the solid line FWDposition in FIG. 14. This maintains a latch mechanism, indicatedgenerally by the reference numeral 182 in position. This latch mechanism182 will now be described by primary reference to FIG. 16.

As seen in FIG. 16, the fixed mounting bracket 178 is formed with afirst, forward locking notch 183 and a second, reverse locking notch184. A sliding latch pin 185 has a pivotal connection, at 186 to theshift bracket 174 adjacent the tension spring 181 and at a locationspaced from the pivot pin 175. This latch pin 185 will be reciprocatedupwardly or downwardly upon pivotal movement of the shifting bracket 174around the pivot pin 175.

A headed fastener 187 extends through a key hole slot 188 formed in thelower portion of the latching pin 185 so as to retain it axially inposition but to permit this reciprocal motion.

A pin 189 is carried by the mounting bracket 178 and extends into anarcuate slot 191 formed in the plate 177. The arc of the slot 191 isaround a radius that is coincident with the axis defined by the pivotpin 179. The length of the slot 191 limits the degree of pivotalmovement of the lever 176 about the pivot pin 179.

In order to ensure smooth shifting, the lever shaft 176 and plate 177have a disconnectable detent connection between them. To this end, theplate 177 is formed with a portion that carries a spring biased plunger192. This plunger 192 can be received in a detent recess 193 formed inthe plate 177 so as to permit some movement of the shift lever 176without accompanying movement of the plate 177. This is done so as topermit some movement until the dog clutching elements 168 engage withthe forward drive gear 155.

Continuing to refer to the shift mechanism and now referring primarilyto FIG. 14, it will be seen that a first shift link 194 is pivotallyconnected at one end to the shift plate 177. The opposite end of theshift link 194 is pivotally connected to a bell crank 195 that isjournaled for pivotal movement on the transmission case 146. A furthershift link 196 is pivotally connected at one end to the bell crank 195and at the other end to a shift yoke 197 which also appears in FIG. 18.This shift yoke 197 is coupled to a shift shaft 198 which is journaledin the transmission housing piece 148 and which carries a shift fork199. The shift fork 199, in turn, has its end portions received in ashift collar 201 that is formed integrally with the reverse gear and dogclutching member 164. Hence, pivotal movement of the shift fork 199moves the reverse gear and dog clutching member 164 between the forwarddrive position shown in solid lines in FIG. 15 and the reverse driveposition shown in phantom lines in this figure.

This shifting operation is achieved by the motions which will now bedescribed by primary reference to FIGS. 14-16 and primarily FIGS. 14 and16. Considering the transmission to be in the forward drive position asshown in solid lines in the figures, the operator first depresses theshift knob 173 so as to pivot the shift rod 172 and shift bracket 174about the pivotal connection 175 to the shift lever 176. This will causethe shift locking pin 185 to be moved from the solid line position shownat the left-hand side of FIG. 16 axially outward of the forward lockingslot 183. During this time, the tension spring 181 will be loaded.

Having thus released the lock, the operator can then pivot the shiftlever 176 and shift plate 177 in a rearward direction which would becounter-clockwise in FIG. 14 and clockwise in FIG. 16. During thisshifting motion, the detents 192 and 193 may be released if there isgreat resistance to shifting. In any event, once the shift is completed,then the locking pin 185 will register with the reverse lock slot 184 ofthe bracket 178. When the operator then releases the pressure on theshift knob 173, the locking pin 185 will be urged by the spring 181 intothe reverse locking slot 184. This position is shown in phantom lines inFIG. 16.

The Shift Assist Brake

As has been noted, the transmission control mechanism 171 alsoincorporates an arrangement for braking the rotation of the intermediatedriving shaft 58 at the times when the transmission is shifted betweenits forward drive position and its reverse drive position. That brakingmechanism will now be described by particular reference to FIGS. 14, 15,and 17.

As has been noted, the snowmobile 31 is provided with a main servicebraking system which service braking system operates on the intermediatedriving shaft 58. This main braking system includes a brake disc orrotor 202 which is affixed for rotation by a keyed connection 203 withthe intermediate driving shaft 58. A service or main braking system,indicated generally by the reference numeral 203 is associated with thisbrake disc 202 for braking the operation of the snowmobile 31 duringnormal operation. This main braking system will be described later.

In addition to the main service brake 203, an auxiliary and parkingbrake caliper assembly, indicated generally by the reference numeral 204also cooperates with the brake disc 202 for braking its rotation andthat of the intermediate drive shaft 58. This caliper 204 includes apair of scissors like caliper members 205 and 206 that are pivotallyconnected to each other by a pivot pin 207. The pivot pin 207 is, inturn, fixed to the mounting bracket 178 so as to be held againstrotation relative to the frame assembly 32.

The caliper leg 205 supports a first brake pad 208 that is disposed onone side of the brake disc 202. The caliper leg 206 supports a secondbrake pad 209 that is disposed on the opposite side of the brake disc202 in confronting relationship to the brake pad 208. A torsional spring211 encircles the pivot pin 207 and urges the brake caliper members 205and 206 to a released position as shown in FIG. 17.

In order to actuate the caliper assembly 204 to its brake position, awire actuator assembly, indicated generally by the reference numeral 212is provided and which cooperates with an extension 210 of the shiftactuating bracket 174 so as to actuate the brake caliper 204 when thelever 174 is pivoted about the pivot pin 175 simultaneously with therelease of the shift latch mechanism 182.

To this end, a wire actuator 213 is affixed, as at 214 to the extension210 of the shift actuating lever 174 at one end of the wire actuator213. The other end of the wire actuator 213 is connected by a ferrule215 to the caliper member 205.

A protective sheath 216 of the wire actuator 212 has one end portion 217affixed to the lever 176 of the shift actuator. The other end of thesheath 216 is affixed, as at 217, to the remaining caliper member 206.

As a result of these connections of the wire actuator 212, when theshift rod 172 and shift lever 174 are pivoted about the pivot pin 175,the wire actuator 213 will be pulled through the sheath 216, and thesheath 216 will be forced in the opposite direction. This causes thecaliper members 205 and 206 to pivot relative to each other in oppositedirections about the pivot pin 207 to compress the torsional spring 211.This action brings the brake pads 208 and 209 into braking relationshipwith the brake disc 202. Hence, before the shift is actually effected,the brake assembly 204 will be energized, and the rotation of thetransmission elements will be braked.

The shift is then executed in the manner aforenoted. After the shift hasbeen completed, the release of the shift rod 172 and shift lever 174will cause the spring 211 along with the operation of the tension spring181 to return the mechanism to its position, and the brake will bereleased as the shift is completed. It should be readily apparent tothose skilled in the art that the brake actuation occurs during theshifting process from forward to reverse, or from reverse to forward.

The Main Throttle and Brake Controls

The throttle control and main braking control for the snowmobile 31 willnow be described by reference to FIGS. 2 and 20, for the actuatormechanisms, and to FIG. 15, for the service brake actuating mechanism.Referring first to FIG. 20, the handlebar assembly 37 is shown in moredetail. This includes a main handlebar 218 that has hand grips 219 and221 at its opposite ends. Associated with the right-hand hand grip 221is a throttle control lever 222.

This throttle control lever 222 is pivotally supported on the end of thehandlebar 218 adjacent the hand grip 221 by a pivot pin 223. A wireactuator 224 is associated with the throttle lever 222 at one end andwith the throttle valves of the carburetors 114 at the opposite end soas to permit the operator to open and close the throttle valves andcontrol the speed of the engine. A suitable return spring is employed sothat when the throttle lever 222 is released, the throttle valves willbe returned to their idle position.

Adjacent the handlebar assembly 219 is the service brake controlmechanism, indicated generally by the reference numeral 225. Thisservice brake control mechanism includes a mounting bracket 226 thatcarries a master cylinder 227. A service brake actuating lever 228 ismounted for pivotal movement on the bracket 226 by a pivot pin 229.Pivotal movement of the service brake lever 228 will pressurize themaster cylinder 227 and direct hydraulic fluid under pressure through abrake line 231.

Referring now to FIG. 15, the opposite end of the brake line 231 isconnected to a caliper assembly 232 of the service brake 203. Theconnection provides hydraulic fluid to a pair of fluid chambers 233 and234 formed on the opposite legs of the caliper assembly 232. Hydraulicpistons in these chambers 233 and 234 act upon brake pads 235 and 236,respectively, which are slidably supported on pins 237 that span thelegs of the caliper assembly 232. When these brake pads 235 and 236 areactuated, they will be forced in opposite directions into engagementwith the brake disc 202 to brake its rotation.

The caliper assembly 232 is fixed relative to the vehicle frame byfasteners 238 which bolt the caliper assembly 232 to the transmissionhousing 146 and, accordingly, to the snowmobile frame 32.

As may be best seen in FIGS. 15 and 19, the area adjacent thetransmission case 146 is formed with a ledge 239 that supports a batterycarrier 241. A storage battery 242 is retained in the battery carrier241 by a strap 243 that is connected at its opposite ends to the frameassembly so as to fix the battery 242 in place. The strap 243 cooperateswith a retainer 244 formed on the battery case 241.

The Parking Brake

The snowmobile 31 is also provided, in addition to the service brake 203and the transmission shift assist brake, a parking brake assembly. Thisparking brake assembly is actuated by a parking brake control levermechanism, indicated generally by the reference numeral 245, as seen inFIGS. 20 and 21, so as to operate the mechanical caliper-type brake 204by effecting counter-pivotal movement of the caliper members 205 and206. This mechanism will now be described by reference to those figures(20 and 21).

The parking brake mechanism includes a mounting bracket assembly 246that is affixed appropriately to the handlebar member 218 adjacent thehand grip 219 and in proximity to the service brake actuating mechanism225. However, this parking brake actuating mechanism 245 is configuredso as to avoid inadvertent operation of the parking brake 245 and toafford a warning to the operator when the parking brake lever 245 is inits braking position, as will become apparent.

The parking brake mechanism actuating lever 245 includes a lever 247which is pivotally mounted on the bracket 246 by a pivot pin 248. Itshould be noted that the pivot axis defined by the pivot pin 248 extendstransversely to the pivot axis of the service brake actuating lever 228defined by the pivot pin 229. In addition, the brake actuating leversare on opposite sides of the handlebar assembly 37.

A wire actuator, indicated generally by the reference numeral 249, has awire portion 251 which is connected by a ferrule and pivot pin 252 tothe parking brake lever 247. The pivot pin 252 is disposed at an offsetlocation to the pivot pin 248 for the lever 247 and provides anover-center relationship, as will be described.

A wire sheath 252 encircles the wire portion 251 and is fixed at one endto the mounting bracket 246. The other end of the wire sheath 252 isaffixed to the caliper member 206 by a fastener assembly 253. It shouldbe noted that this connection for the wire actuator 249 is locatedradially outwardly from the pivot pin 207 of the caliper assembly 204from the connection of the wire actuator 212. This provides a greatermechanical advantage. It is important that the parking brake mechanismexert more force on the caliper 204 than the transmission brakingmechanism, for obvious reasons.

The opposite end of the wire actuator 251 is connected by means of aferrule 254 to the remaining caliper member 205.

The parking brake actuating lever 247 is provided with a folded-over endportion 255 that extends transversely to its main portion and which thusprovides a part that will be easy for the operator to access, eventhough he is wearing heavy gloves. This off-turned portion 255 is,however, configured so as to provide a neat appearance and to avoidunnecessary protrusion.

As seen in FIG. 21, when the parking brake lever 247 is in its releasedposition, as shown in solid lines, the pivot pin 252 will be disposed onone side of the pivot axis 248 of the lever 247. Hence, there will be anover-center relationship established that will tend to retain theparking brake lever 245 in this position.

When the operator wishes to engage the parking brake, the lever 247 isrotated, as shown by the arrow in FIG. 21, to the phantom line orengaged position. When this occurs, a tensile force will be placed onthe wire actuator 251 so as to pivot the caliper member 205 in acounter-clockwise direction, as seen in FIG. 21. At the same time, aforce will be exerted through the wire sheath 252 on the caliper member206 to cause it to rotate in the clockwise direction. Hence, the brakepads 208 and 209 will be forced into engagement with the disk 202 tobrake its rotation.

In this position, the pivot pin 252 moves to the other side of thesupporting pivot pin 248 for the parking brake lever 247. In thiscondition, a further over-center relationship is established that willretain the parking brake in its engaged position.

Also, it should be noted that in this position, the parking brake lever247 will be readily visible to the operator. In addition, if he attemptsto put his hand on the handle grip 219, he will notice the presence ofthe parking brake lever 247 in its engaged position. Thus, thelikelihood of inadvertent attempts to operate the snowmobile 31 with theparking brake assembly engaged will be avoided.

Finally, in connection with the handlebar and control assembly, itshould be noted that the central portion of the handlebar 218 is coveredby a protective covering 256. This protective covering covers the centerportion of the handlebar assembly 218 between the throttle controlmechanism 222 and the brake controlling mechanisms 225 and 245. Thus, aneat appearance is provided. Again, however, the off-turned portion 255of the parking brake lever 247 will permit a rider with gloves to movethe parking brake 247 between its release to its engaged positions, evenwhen wearing thick gloves.

The Engine Cooling System

As has been noted, the engine 56 is of the water-cooled type. Coolingliquid, such as an anti-freeze type coolant, is provided in the coolingjacket of the engine 56. This coolant is circulated through a heatexchanging system, shown in FIGS. 22-36, for exchanging heat between theengine and the atmosphere to keep the engine 56 at its desired operatingtemperature. This cooling system includes a number of heat exchangerswhich are interrelated with the frame assembly 32, and moreparticularly, the rear frame subassembly 33 and front frame subassembly34 so as to provide not only adequate cooling, but also to add to therigidity of the frame structure. This permits a lightening in weight ofthe frame without any structural sacrifices.

This heat exchanger system includes a front heat exchanger, indicatedgenerally by the reference numeral 257, a pair of side heat exchangers,indicated generally by the reference numerals 258 and 259, and a rearheat exchanger, indicated generally by the reference numeral 261. As maybe seen by reference to FIGS. 22 and 23, the heat exchangers 257, 258,259 and 261 are disposed in proximity to, but surrounding relationshipwith, the drive belt 48.

In addition, the front and rear heat exchangers 257 and 261 span theframe assembly from side-to-side, and thus provide reinforcing as willbe described. In a like manner, the side heat exchangers 258 and 259underlie and are, in effect, integrally connected to the footstepportions 63 and 64 of the rear frame subassembly 33, so as to furtherrigidify this construction. The construction of the individual heatexchangers will be described first, and then their relationship to theframe assembly and the overall cooling system will be described.

Referring first primarily to FIGS. 24 and 25, the front heat exchanger257 will be described. This heat exchanger 257 includes a main bodyportion 262 which is formed as an extrusion from a lightweight materialhaving a high coefficient of conductivity, such as aluminum or analuminum alloy. This extrusion 262 is formed with a lower flange portion263 and an upper flange portion 264. These flange portions 263 and 264are formed with a plurality of openings that pass threaded fasteners orrivets so that the heat exchanger 257 can be rigidly affixed in theframe assembly, and particularly extending across an opening formed inthe front frame subassembly 34 below the inclined wall 76, as seen inFIG. 7. Thus the heat exchanger 257 extends transversely across and alsoto provide reinforcing for this area of the frame assembly. Otherrelationships between the heat exchanger 257 and the other components ofthe snowmobile will be described later.

The interior portion of the extrusion 262 is divided by a plurality ofinternal walls 265, 266 and 267 into parallel extending flow chambers268, 269 and 271. Cooling fins 272 extend rearwardly from these chambersinto the frame assembly in an area where they will experience air flow,and thus afford cooling for the coolant circulated therethrough.

The ends of these chambers 268, 269 and 271 are closed by end plates 273and 274 so as to provide a water-tight closure for these flow paths.These ends plates 273 and 274 are also formed with openings so as topermit attachment to the sides 68 and 69 of the front frame subassembly34 so as to further rigidify this structure.

An L-shaped water coolant inlet pipe 275 is formed at one side of theextrusion 262 in communication with the left-hand side of the lowerwater channel 268. The wall 265 extends uninterruptedly from the endplate 273 and terminates at an end portion 276 (FIG. 25) that is spacedfrom the end plate 274. Hence, the coolant which is flowing from left toright through the channel 268 may then flow upwardly into the channel269. This coolant then can flow transversely back across the extrusion262 toward the end plate 273.

The wall 266 terminates at an end portion 277 that terminates short ofthe end plate 273 so that coolant that has passed along the channel 269may then flow upwardly into the channel 271. The coolant then reversesits flow and flows back to an outlet tube 279 that is affixed to thefront side of the extrusion 262 and which delivers the coolant, in amanner to be described, to the engine water pump.

As may be best seen in FIGS. 7, 24 and 25, the forward portion of thefront heat exchanger 257 extends through an opening in the frame member76. The front of the extrusion 262 is provided with an upper flange 281and a lower flange 282. The flanges 281 and 282 are integrally connectedto the extrusion 262 and define a cavity for holding a steering shaftsupport assembly, indicated generally by the reference numeral 283.

This support assembly 283 is comprised of a casting or forging 284 whichis formed from an aluminum or aluminum alloy. This member 284 isreceived within the area bounded by the flanges 281 and 282 and isrigidly affixed thereto as by weld beads 285 and 286. This thus addsrigidity to the heat exchanger 262 and the frame assembly.

A pair of threaded fasteners 287 are carried by this member 284 andafford attachment for the bearing assembly 106 which forms theaforenoted journal for the lower end of the steering shaft 105. Thus,this assembly for the front heat exchanger 257 provides not onlyadequate and efficient cooling for the engine 56, but also adds to therigidity of the frame assembly and permits it to function to journal thelower end of the steering shaft 105.

The construction of the side heat exchangers 258 and 259 will now bedescribed by primary reference to FIG. 26, although this structure alsois shown in the other figures showing the cooling system. The heatexchangers 258 and 259 are actually identical in construction, but oneis reversed relative to the other. The reason for this will bedescribed.

Each heat exchanger is comprised of a generally L-shaped assembly havinga vertically extending leg 289 and a horizontally extending leg 291.These legs are apertured so as to permit connection by rivets 292, orthe like, to a vertically extending walls 62 that connect the frameportion 61 with the frame portions 63 and 64. The leg 291 is affixed tothe underside of the footrest portions 63 and 64, and thus, the heatexchangers 258 and 259 further rigidify the frame assembly 32,particularly in this critical area.

The lower leg 291 is hollow and is divided by wall portions 294 and 295into three flow channels 296, 297 and 298. The ends of these flowchannels are closed by a suitable closure plate. Like the walls 265, 266and 267 of the front heat exchanger 257, the walls 294 and 295 terminateshort of one end so as to provide a serpentine flow path for the coolantthrough the heat exchangers 258 and 259.

Continuing to refer primarily to the heat exchanger 259, one end of itis provided with a coolant inlet fitting 299. The inlet fitting 299 ofthe heat exchanger 259 is at the front of the snowmobile, while thecorresponding inlet fitting 299 of the heat exchanger 258 is at the rearend. These fittings 299 communicate at one end of the flow channel 298.

The wall 295 that defines this flow channel terminates short of the endportion so as to provide a crossover passage 301 so that coolant thathas entered one end of the channel 298 can flow transversely across tothe other end of the channel 297.

The coolant then flows along the channel 297 to a point where the wall294 terminates short of the inlet end of the channel 298. This permitsthe coolant to then cross over to the remaining flow channel 296 andflow back toward the opposite end. A coolant discharge fitting 302 isformed therein that permits discharge of the coolant from the respectiveside heat exchanger 258 or 259. The discharge fitting 302 of the heatexchanger 259 is to the rear, while the outlet 302 of the heat exchanger258 is to the front.

The rear heat exchanger 261 is shown only in FIGS. 22 and 23. This heatexchanger 261, like the heat exchangers 257, 258 and 259, is formed of amain body portion 303 that may be formed from an aluminum extrusion andwhich has internal baffles so as to define a serpentine three pass flowpath. This flow path begins at an inlet fitting 304 formed at the sideadjacent the side heat exchanger 259. From the inlet fitting 304, thecoolant flows transversely across toward the opposite side of thesnowmobile.

The coolant flow then reverses direction and flows back toward the inletside, as with the previously described heat exchangers. Finally, theflow path passes back in the same directly as the inlet, and exitsthrough a discharge fitting 305 associated with the left-hand side ofthe snowmobile and adjacent the side heat exchanger 258.

These flow paths are closed by end plates 306 that are affixed toopposite ends of the extrusion 303 and which afford a means forattachment to the side walls 293 of the frame subassembly 33. As aresult, this closure rigidifies the rear end of this subassembly 33, andthe heat exchanger 261, in fact, forms a portion of this framesubassembly.

The heat exchanger 261 is to the rear of the drive belt 48, even in thefully compressed state thereof, as shown in FIG. 22, so as to affordadequate clearance and protection. Cooling fins (not shown) may beprovided on both the front and rear portions of the rear heat exchanger61.

Referring to FIG. 1, it should be seen that the snowmobile 31 isprovided with a rear bumper assembly 307 which is juxtaposed to the rearheat exchanger 261 and which will protect it. A flap 308 depends fromthe body in this area to further control the air flow and snow which maybe thrown rearwardly by the drive belt 48.

The coolant path through the entire system will now be described byreference to FIGS. 22 and 23. The engine 56 is provided with a crankcaseassembly 309 upon which a water pump 311 is mounted, and which is drivenfrom the engine in a suitable manner. A short flexible hose 312 deliverswater from the front heat exchanger outlet 279 to this water pump 311.The water is then circulated through the engine, and is discharged froma housing assembly 313, which may include a thermostat, to another shortdelivery hose 314. A fitting 315 is provided in this hose 314. Thisfitting 315 has a fill neck that is closed by a cap 316 so that coolantcan be added to the system.

Another short hose 317 extends from the fitting 315 to the inlet fitting299 of the side heat exchanger 259. After the coolant has passed throughthe three passes of the side heat exchanger 259, it is discharged fromthe discharge fitting 311 to a short connecting hose 318 into the rearheat exchanger inlet fitting 304.

The coolant then flows through the three passes of the rear heatexchanger 261 and is discharged therefrom through its outlet fitting305. A short flexible conduit 319 connects the rear heat exchangeroutlet fitting 305 with the inlet fitting 299 of the remaining side heatexchanger 258. The coolant can then flow through the three passes ofthis heat exchanger and be discharged from its outlet fitting 302.

The outlet fitting 302 of the side heat exchanger 258 is connected by ashort flexible connecting pipe 321 to the inlet fitting 275 of the frontheat exchanger 257.

Thus, it should be readily apparent that the cooling system isconfigured so as to provide a very large surface area for heat exchangeand also a long flow path for the coolant. The heat exchangers, asalready noted, also rigidify the frame assembly and perform certainadditional structural functions, as in the case of the front heatexchanger 257. All of this is accomplished with very short connectinghoses, and hence, the likelihood of leakage or hose damage issubstantially reduced.

An expansion tank 322 is mounted forwardly of the fuel tank 38 foroverflow purposes. This expansion tank 322 receives overflow coolantthrough a conduit 322 that interconnects with the fitting 315 in thearea of the cap 316. A fill neck closed by a cap 324 is also provided onthe upper end of the expansion tank 322.

In this embodiment as thus far described, the side heat exchangers 258and 259 have been formed from aluminum extrusions, with internal wallsthat define their three pass flow paths. Of course, other types of heatexchangers may be employed. FIG. 27 shows another embodiment which isbasically the same as the embodiment thus far described. Thisembodiment, however, shows how the invention can be utilized with atwo-cylinder engine, and the side heat exchangers, indicated by thereference numerals 501 and 502, are provided with internal flow pathsthat are defined by finned and suitable apertured baffle plates 503, todefine the flow paths through their internal flow channels 504. In allother regards, this embodiment is the same as those previously describedand, for that reason, further description of this embodiment is notbelieved to be necessary to permit those skilled in the art to practicethis invention.

Thus, the foregoing description it should be readily apparent that thesnowmobile 31, and particularly the exhaust system therefor, permits avery short, compact exhaust system. The exhaust system, and specificallythe exhaust manifold, is provided with internal expansion chambers thatcommunicate through restricted openings with the branch pipes so as tominimize adverse effects of pulse-back, and to obtain improvedperformance, not only in the low-speed range, but through the entireengine speed and load ranges. Of course, the foregoing description isthat of preferred embodiments of the invention, and various changes andmodifications may be made without departing from the spirit and scope ofthe invention, as defined by the appended claims.

What is claimed is:
 1. An exhaust system for an internal combustionengine having at least two combustion chambers, each formed with anexhaust port through which exhaust gases are discharged from saidcombustion chambers to the atmosphere, said exhaust system comprising apair of spaced-apart tubular exhaust branch pipes extending from saidexhaust ports to a discharge end, means affixed to said exhaust branchpipes and defining therewith a closed expansion chamber in directsurface contact with outer surface portions of each of said exhaustbranch pipes, said outer surface portions of said branch pipes defininga peripheral portion of said expansion chamber, and a passage extendingthrough at least the portion of one of said exhaust branch pipes forproviding communication of the exhaust gases from said at least oneexhaust branch pipe into said expansion chamber.
 2. An exhaust system asset forth in claim 1, wherein passages are formed in each of the exhaustbranch pipes communicating with an expansion chamber.
 3. An exhaustsystem as set forth in claim 2, wherein the exhaust branch pipes bothcommunicate with the same expansion chamber.
 4. An exhaust system as setforth in claim 1, wherein the branch pipes are formed from joinedtubular semi-cylindrical shells, and the means affixed to the branchpipes comprises a pair of plates affixed to surfaces of the shells anddefining between the shells and said plates the expansion chamber.
 5. Anexhaust system as set forth in claim 4, wherein passages are formed ineach of the exhaust branch pipes communicating with an expansionchamber.
 6. An exhaust system as set forth in claim 5, wherein theexhaust branch pipes both communicate with the same expansion chamber.7. An exhaust system as set forth in claim 1, wherein there arecomprised three exhaust branch pipes comprised of a pair of end exhaustbranch pipes and a center exhaust branch pipe, all connected to a commonoutlet.
 8. An exhaust system as set forth in claim 7, wherein the meansaffixed to the exhaust branch pipe define a pair of expansion chambers.9. An exhaust system as set forth in claim 8, wherein passages areformed in the center exhaust branch pipe that communicates with each ofthe expansion chambers.
 10. An exhaust system as set forth in claim 9,wherein the portions of the walls of the branch pipes define aperipheral portion of the respective expansion chamber.
 11. An exhaustsystem as set forth in claim 10, wherein the branch pipes are formedfrom joined tubular semi-cylindrical shells, and the means affixed tothe branch pipes comprises a pair of plates affixed to surfaces of theshells and defining between the shells and said plates the expansionchambers.